In an automotive vehicle in recent years, an engine automatic stop and start system, which is generally referred to as an idle stop system (idle reduction system), is provided to automatically stop an engine when a predetermined stop condition is satisfied and thereafter automatically start the engine when a predetermined start condition is satisfied (for example, patent document 1).
The patent document 1 discloses a starter, in which a pinion gear driven to rotate by a motor is switchable to a state of engagement with a ring gear of the engine or a state of disengagement from the ring gear independently of current supply or no current supply to a motor of the starter, that is, irrespective of energization or deenergization of the motor. This kind of motor, in which the motor and the pinion gear are controllable independently, is referred to as an independently-controlled starter. Such an independently controlled starter may be provided in a vehicle (idle stop vehicle), which is provided with the idle stop system.
More specifically, the pinion gear is driven to rotate by the starter motor while being engaged with the ring gear of the engine so that the engine is cranked by rotation of the ring gear of the engine. A pinion control solenoid and a pinion control relay are provided separately from a relay, which switches over energization and deenergization of the starter motor. The pinion control solenoid is for driving the pinion gear for engagement with the ring gear of the engine. The pinion control relay is located upstream the pinion control solenoid (more specifically, upstream of a coil of the pinion control solenoid) for switching over energization and deenergization of the coil. A solenoid coil is generally referred to as a solenoid in the field of electric technology. In this description, however, a solenoid is also referred to as an actuator, which includes a coil and movable parts driven by electromagnetic force of the coil in the field of electromechanical technology.
In the patent document 1, the pinion gear is engaged with the ring gear by turning on the pinion control relay to energize the coil of the pinion control solenoid after the engine is automatically stopped by the function of the idle stop system. For restarting the engine, the starter motor is energized to crank the engine with the ring gear and the pinion gear having been already engaged with each other. Thus wear of the pinion gear of the starter and the ring gear of the engine is reduced. In addition, noise sound and vibration generated by engagement of the gears are also reduced. Since the engine is automatically stopped and automatically started during a travel (including a state of zero vehicle speed) in the idle stop vehicle, it is more likely that the independently controlled starter is used to reduce wear, noise sound and vibration.
Patent document 2 discloses detection of failure of a starter, which is different from the independently controlled starter, by measuring a contact resistance between electrical contacts in an electric circuit provided for driving a motor of the starter.    [Patent document 1] JP H11-30139A    [Patent document 2] JP 2008-111343A
In an independently controlled starter, position of a pinion gear cannot be switched over (that is, switching between states of engagement disengagement with a ring gear) even in a case that abnormality arises in not only a current supply path to the motor but also in a current supply path to the coil of the pinion control solenoid. This may cause some troubles in starting an engine.
In both patent documents 1 and 2, no abnormality is detected in the current supply path to the coil of the pinion control solenoid and no countermeasure for such abnormality is proposed.
Since a contact resistance between electrical contacts is measured according to technology of the patent document 2, configuration for detecting abnormality becomes complicated. For example, a measuring device for measuring the resistance need be provided in the starter and the measuring device need be a high-precision device.